Led display device

ABSTRACT

A semiconductor light emitting device has a double heterostructure. The device is composed of an active layer and clad layers that sandwich the active layer. At least one of the clad layers has a multilayer structure having at least two element layers. The Al mole fraction of an element layer, which is proximal to the active layer, of the multilayer structure is smaller than that of the other element layer thereof distal from the active layer. This arrangement improves the crystal quality of an interface between the active layer and the clad layer of multilayer structure and effectively confines carriers in the active layer.

This is a division of application Ser. No. 08/611,460, filed Apr. 11,1996, now U.S. Pat. No. 5,732,098.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-power semiconductor lightemitting diode (referred to hereinafter as “LED”) display device, andparticularly, to a LED display device having a LED matrix circuit for anoutdoor/indoor display panel, a railway sign board, a traffic signboard, or a vehicle-mounted light.

2. Description of the Prior Art

Semiconductor light emitting devices such as LEDs and semiconductorlasers are manufactured according to a liquid-phase epitaxial growth(LPE) technique and a vapor-phase epitaxial (VPE) growth technique suchas a metal organic chemical vapor deposition (MOCVD) technique. Any ofthe techniques forms a double hetero (DH) structure to confine carriersin an active layer serving as a light emitting layer and realize highbrightness.

FIG. 1 is a sectional view showing an InGaAlP-based LED having a DHstructure according to a prior art. Successively laminated on an n⁺-typeGaAs substrate 1 are an n-type GaAs buffer layer 2, an n-typeIn_(0.5)Al_(0.5)P/GaAs reflection layer (quarter-wave stack mirror) 3,an n-type In_(0.5)(Ga_(1−x)Al_(x))_(0.5)P clad layer 41, an undopedIn_(0.5)(Ga_(1−y)Al_(y))_(0.5)P active layer 61, and a p-typeIn_(0.5)(Ga_(1−x)Al_(x))_(0.5)P clad layer 81. Here, x≦1 and x>y.

On a part of the clad layer 81, an n-type GaAs current block layer 91 isformed. On the current block layer 91 and clad layer 81, a p-typeGa_(0.3)Al_(0.7)As current diffusion layer 10 is formed. On a part ofthe current diffusion layer 10, a p⁺-type GaAs contact layer 11 isformed. On the contact layer 11, a p-type electrode 13 is formed. On thebottom surface of the substrate 1, an n-type electrode 12 is formed.

The structure of FIG. 1 is epitaxially grown according to a low pressureMOCVD (LPMOCVD) technique that employs trimethylindium (TMI),trimethylgallium (TMG), and trimethylaluminum (TMA) as materials ofgroup III, arsine (AsH₃) and phosphine (PH₃) as materials of group V,silane (SiH₄) and dimethylzinc (DMZ) as doping materials, and hydrogenas a carrier gas. These materials epitaxially grow crystals under a lowpressure. More precisely, a wafer having an n-type GaAs substrate 1 isplaced in a CVD reactor and is kept at a given pressure and temperature.A mass flow controller supplies the group III, V, and doping materialsinto the reactor at set flow rates, to epitaxially grow layers one afteranother on the substrate 1. Namely, an n-type GaAs buffer layer 2, ann-type In_(0.5)Al_(0.5)P/GaAs reflection layer 3, an n-typeIn_(0.5)(Ga_(1−x)Al_(x))_(0.5)P clad layer 41, an undopedIn_(0.5)(Ga_(1−y)Al_(y))_(0.5)P active layer 61, a p-typeIn_(0.5)(Ga_(1−x)Al_(x))_(0.5)P clad layer 81, and an n-type GaAscurrent block layer 91 are successively formed on the substrate 1. Here,x≦1 and x>y.

The wafer with the layers is taken out of the reactor, and the currentblock layer 91 is selectively etched according to a photolithographytechnique into the shape of FIG. 1. The MOCVD method is again employedto form a p-type Ga_(0.3)Al_(0.7)As current diffusion layer 10 and ap--type GaAs contact layer 11.

Au-based material is deposited on each surface of the wafer according toa vacuum evaporation technique. The Au-based layer is selectively etchedaccording to the photolithography technique to form a p-type electrode13 as shown in FIG. 1. The contact layer 11 is selectively etched topartly expose the current diffusion layer 10. An n-type electrode 12 isformed over the bottom surface of the substrate 1. The wafer Is dicedinto chips that serve each as the semiconductor light emitting device ofFIG. 1. Each chip is mounted on a stem, bonded, sealed with resin, andfabricated into a φ 5-mm lamp.

FIG. 2 is a graph showing the initial brightness I_(o) and remnantbrightness ratio η of the φ 5-mm lamp. The remnant brightness ratio isthe ratio of the brightness I₁₀₀₀ of the lamp measured after 1000 hoursof operation at 50 mA to the initial brightness I_(o). Namely,η=(I₁₀₀₀/I_(o))×100. Here, the mole fraction “y” of Al of the activelayer 61 is 0.3, and the mole fraction “x” of Al of each of the cladlayers 41 and 81 is changed among 1.0, 0.9, 0.8, and 0.7.

When the Al mole fraction “x” of In_(0.5)(Ga_(1−x)Al_(x))_(0.5)P of theclad layers 41 and 81 is increased, the initial brightness I_(o)increases but the remnant brightness ratio η decreases. When the Al molefraction “x” is decreased, the initial brightness I_(o) decreases butthe remnant brightness ratio η increases. In this way, the initialbrightness I_(o) and remnant brightness ratio η are trade-offs. It isdifficult for the conventional DH structure to provide high brightnessas well as long service life.

FIG. 3 shows an InGaAlP-based LED having a DH structure according toanother prior art. Sequentially laminated on an n⁺type GaAs substrate 1are an n-type GaAs buffer layer 2, an n-type In_(0.5)Al_(0.5)P/GaAsreflective multilayer 3, an n-type In_(0.5)Al_(0.5)P clad layer 42, ann-type In_(0.5)(Ga_(0.72)Al_(0.28))_(0.5)P active layer 62, a p-typeIn_(0.5)Al_(0.5)P clad layer 82, a p-type In_(0.5)Ga_(0.5)P contactlayer 127, and a p-type GaAs protection layer 128. On a part of theprotection layer 128, there are sequentially laminated an n-typeIn_(0.5)(Ga_(0.3)Al_(0.7))_(0.5)P current block layer 92, an n-type GaAsprotection layer 93, a p-type Ga_(0.3)Al_(0.7)As current diffusion layer10, a p-type In_(0.5)(Ga_(0.7)Al_(0.3))_(0.5)P diffusion layer 132, anda p⁺-type GaAs contact layer 11. The contact layer 11 is formed on apart of the diffusion layer 132. On the contact layer 11, a p-typeelectrode 13 is formed. An n-type electrode 12 is formed on the bottomsurface of the substrate 1.

When designing the clad layers 42 and 82 that sandwich the active layer62 serving as a light emitting layer, the following opposing factorsmust be considered:

(a) To confine a sufficient quantity of minority carriers in the activelayer 62, the band gap (Eg) of the clad layers 42 and 82 must beproperly larger than that of the active layer 62. Namely, the Al molefraction “x” of the clad layers 42 and 82 must be large.

(b) Crystal defects that trap minority carriers must be minimized ineach interface between the clad layers 42 and 82 and the active layer62. Such defects, however, easily occur in the interfaces where the Almole fraction of the clad layers 42 and 82 greatly differs from that ofthe active layer 62. To reduce the crystal defects, the Al mole fraction“x” of the clad layers 42 and 82 must be small.

When the Al mole fraction of the clad layers 42 and 82 greatly differsfrom that of the active layer 62 that emits yellow light, initialbrightness may be high but there will be many crystal defects in eachinterface among the layers. As a result, normalized light intensityP/P_(o) deteriorates in proportion to an operating time as shown in FIG.4.

The Al mole fraction of the clad layers of the conventional DH structureLED must be high to provide high brightness when the LED is usedoutdoors. This, however, causes many crystal defects to shorten theservice life of the LED. If the Al mole fraction of the clad layers islow to extend the service life of the LED, the light output of the LEDwill be low, and therefore, the brightness thereof will be improper foroutdoor use. In this way, the light output and service life of theconventional LED are trade-offs.

The conventional DH structure semiconductor light emitting devices areincapable of simultaneously providing high brightness and long servicelife.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor lightemitting device having a DH structure that realizes high brightness aswell as long service life.

In order to accomplish the object, the present invention provides asemiconductor light emitting device having any one of DH structures ofFIGS. 5 to 9 and 12A. An active layer 61 (26) emits light of a givenwavelength and is sandwiched between p- and n-type clad layers whoseforbidden band gap Egc is larger than that (Ega) of the active layer.The Al mole fraction of a clad layer proximal to the active layer issmall, and that of a clad layer distal from the active layer is large.More precisely, at least one of the p- and n-type clad layers has amultilayer structure in which a layer proximal to the active layer has alower Al mole fraction than that distal from the active layer. Namely,the clad layers 44 (46, 25) and 83 (85, 27) proximal to the active layerhave a small forbidden band gap Egc1, and the clad layers 43 (45, 24)and 84 (86, 28) distal from the active layer have a larger forbiddenband gap Egc2 than Egc1.

The clad layers according to the present invention are multi-elementcompound semiconductor layers containing Al, and the Al mole fractionand thickness of each of the clad layers are optimized to realize highbrightness and long service life. The multiple clad layers are calledthe first, second, and the like layers starting from the one closest tothe active layer.

In FIG. 5, an active layer 61 is sandwiched between a first n-type cladlayer 44 and a first p-type clad layer 83, and these layers 61, 44, and83 are sandwiched between a second n-type clad layer 43 and a secondp-type clad layer 84. FIG. 6 shows a DH structure LED that has only twon-type clad layers 44 and 43. FIG. 7 shows a DH structure LED that hasonly two p-type clad layers 83 and 84. In FIG. 8, an active layer 26 issandwiched between two n-type clad layers 25 and 24 and two p-type cladlayers. 27 and 28. In FIG. 9, an active layer 61 is sandwiched betweentwo n-type clad layers 46 and 45 and two p-type clad layers 85 and 86.In each of these devices, the Al mole fractions and thicknesses of thefirst, second, and the like clad layers are optimized to improve thecrystal quality of each interface between the clad layers and the activelayer and effectively confine carriers in the active layer. In FIGS. 12Aand 12B, the Al mole fraction of first clad layers 48 and 88 isgradually increased in proportion to the distance from an active layer61.

When an active layer of In_(0.5)(Ga_(1−x)Al_(x))_(0.5)P is employed, theAl mole fraction “y” of In_(0.5)(Ga_(1−y)Al_(y))_(0.5)P of first p- andn-type clad layers is set as follows:

x+0.1≦y≦x+0.5  (1)

In this case, the thickness of each first clad layer is in the range of0.005 to 0.1 μm.

This structure improves the crystal qualities of each interface betweenthe active layer and the clad layers, to effectively confine carriers inthe active layer and realize high brightness and long service life.

Other and further objects and features of the present invention willbecome obvious upon an understanding of the illustrative embodimentsabout to be described in connection with the accompanying drawings orwill be indicated in the appended claims, and various advantages notreferred to herein will occur to one skilled in the art upon employingof the invention in practice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a semiconductor light emitting devicehaving a DH structure according to a prior art;

FIG. 2 is a graph showing the Al mole fractions of clad layers, initialbrightness, and remnant brightness of the device of FIG. 1;

FIG. 3 is a sectional view showing a semiconductor light emitting devicehaving a DH structure according to another prior art;

FIG. 4 is a graph showing the operating time of the device of FIG. 3;

FIG. 5 is a sectional view showing a semiconductor light emitting deviceaccording to a first embodiment of the present invention;

FIG. 6 is a sectional view showing a semiconductor light emitting deviceaccording to a second embodiment of the present invention;

FIG. 7 is a sectional view showing a semiconductor light emitting deviceaccording to a modification of the second embodiment;

FIG. 8 is a sectional view showing a semiconductor light emitting deviceaccording to a third embodiment of the present invention;

FIG. 9 is a sectional view showing a semiconductor light emitting deviceaccording to a fourth embodiment of the present invention;

FIG. 10A is a graph showing a comparison between the light output of thedevice of the fourth embodiment and that of the prior art of FIG. 3;

FIG. 10B is a graph showing the operating time of the device of FIG. 9;

FIGS. 11A to 11E are sectional views showing the processes ofmanufacturing the device of FIG. 9;

FIG. 12A is a sectional view showing a semiconductor light emittingdevice according to a fifth embodiment of the present invention;

FIG. 12B is a graph showing a change in the Al mole fraction of a firstclad layer of the device of FIG. 12A;

FIGS. 13A to 13D show changes in the Al mole fraction in clad layersaround an active layer according to the fifth embodiment;

FIGS. 14A and 14B show LED matrix circuits each having an X-Y matrix ofthe semiconductor light emitting devices of the present invention; and

FIG. 15 shows an LED module panel having LED matrix circuits arrangedside by side each being any one of those of FIGS. 14A and 14B, andcomponents for driving and controlling the panel.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Various embodiments of the present invention will be described withreference to the accompanying drawings. It is to be noted that the sameor similar reference numerals are applied to the same or similar partsand elements throughout the drawings, and the description of the same orsimilar parts and elements will be omitted or simplified.

Generally and as it is conventional in the representation ofsemiconductor light emitting devices, it will be appreciated that thevarious drawings are not drawn to scale from one figure to another norinside a given figure, and in particular that the layer thicknesses arearbitrarily drawn for facilitating the reading of the drawings.

(First embodiment)

FIG. 5 is a sectional view showing a DH structure light emitting diode(DH-LED) according to the first embodiment of the present invention.Sequentially formed on an n-type GaAs substrate 1 are an n-type GaAsbuffer layer 2, an n-type InAlP/GaAs reflection layer 3, a second n-typeInGaAlP clad layer 43, a first n-type InGaAlP clad layer 44, an undopedInGaAlP active layer 61, a first p-type InGaAlP clad layer 83, and asecond p-type InGaAlP clad layer 84. On a part of the clad layer 84, ann-type GaAs current block layer 91 is formed. On the current block layer91, there are formed a p-type GaAlAs current diffusion layer 10 and ap-type GaAs contact layer 11. The reflection layer 3 is a Bragg mirrorhaving a multilayer structure composed of alternating 10 to 20 pairs ofn-type InAlP and n-type GaAs layers each of λ×(¼n) thick, where A is thewavelength of light emitted from the active layer 61 and n is arefractive index of the material of the reflection layer 3. Thereflection layer 3 reflects light that goes toward the substrate 1 fromthe active layer 61, so that the light effectively runs only toward thecurrent diffusion layer 10.

The Al mole fraction of each of the clad layers 44, 43, 83, and 94 andcurrent diffusion layer 10 is determined to sufficiently transmit thelight from the active layer 61. On the bottom surface of the substrate1, an n-type electrode 12 is formed from, for example, Au—Ge alloy. Onthe contact layer 11, a p-type electrode 13 is formed from, for example,Au—Zn alloy. As is apparent in FIG. 5, the DH-LED of the firstembodiment has two clad layers on each side of the active layer 61 sothat the forbidden band gap Egc of the clad layers increases step bystep in proportion to the distance from the active layer 61.

The Al mole fraction “x” of the second n-typeIn_(0.5)(Ga_(1−x)Al_(x))_(0.5)P clad layer 43 is 1.0, the Al molefraction “y” of the first n-type In_(0.5)(Ga_(1−y)Al_(y))_(0.5)P cladlayer 44 is 0.7, the Al mole fraction “z” of the first p-typeIn_(0.5)(Ga_(1−z)Al_(z))_(0.5)P clad layer 83 is 0.7, and the Al molefraction “w” of the second p-type In_(0.5)(Ga_(1−w)Al_(w))_(0.5)P cladlayer 84 is 1.0. In this way, the Al mole fractions of the clad layersare changed step by step.

The DH-LED of FIG. 5 is mounted on a stem, bonded, and sealed withresin, to form a φ5-mm lamp. The initial brightness I_(o) of this lampis equal to the highest level (a relative value of 750) of the prior artof FIG. 2, and the remnant brightness ratio (η=I₁₀₀₀/I_(o)) ofbrightness I₁₀₀₀ after 1000 hours of operation at 50 mA is 85%, which isfar greater than that of the prior art.

The high quality of the DH-LED of FIG. 5 is derived from the two cladlayers on each side of the active layer 61, with the Al mole fraction ofthe first clad layers 44 and 83 proximal to the active layer 61 beinglow to improve the crystal quality of each interface between the activelayer 61 and the first clad layers, and with the Al mole fraction of thesecond clad layers 43 and 84 distal from the active layer 61 being highto effectively confine carriers in the active layer 61. As a result, theDH-LED of the first embodiment provides higher brightness and longerservice life than the prior art.

The processes of manufacturing the DH-LED of the first embodiment willbe explained.

a) Firstly prepared is an n-type GaAs substrate I having an impurityconcentration of 0.4×10¹⁸ cm⁻³ to 4×10¹⁸ cm³ and a (100) plane shiftedtoward [011] by several degrees. On the substrate 1, an n-type GaAsbuffer layer 2 of about 0.5 μm thick having an impurity concentrationN=4×10¹⁷ cm⁻³ is formed. On the buffer layer 2, a reflection layer 3 isformed by alternately growing 10 to 20 pairs of n-type In_(0.5)Al_(0.5)Pfilm (N=4×10¹⁷ cm⁻³) and GaAs film (N=4×10¹⁷ cm⁻³) of λ/4n thick.Sequentially grown on the reflection layer 3 are a second n-typeIn_(0.5)(Ga_(1−x)Al_(x))_(0.5)P clad layer 43 (N=4×10¹⁷ cm⁻³, x=1.0) ofabout 0.59 μm thick, a first n-type In_(0.5)(Ga_(1−y)Al_(y))_(0.5)P cladlayer 44 (N=4×10¹⁷ cm⁻³, y=0.7) of about 0.01 μm thick, an undopedIn_(0.5)(Ga_(0.7)Al_(0.3))_(0.5)P active layer 61 of about 0.6 μm thick,a first p-type In_(0.5)(Ga_(1−z)Al_(z))_(0.5)P clad layer 83 (N=4×10¹⁷cm⁻³, z=0.7) of about 0.01 μm thick, a second p-typeIn_(0.5)(Ga_(1−w)Al_(w))_(0.5)P clad layer 84 (N=4×10¹⁷ cm⁻³ w=1.0) ofabout 0.59 μm thick, and an n-type GaAs current block layer 91 (N=2×10¹⁸cm⁻³) of about 0.02 μm thick.

The multilayer structure from the buffer layer 2 to the current blocklayer 91 is formed according to the LPMOCVD technique that employstrimethylindium (TMI), trimethylgallium (TMG), and trimethylaluminum(TMA) as materials of group III, arsine (AsH₃) and phosphine (PH₃) asmaterials of group V, and silane (SiH₄) and dimethylzinc (DMZ) as dopingmaterials. More precisely, the substrate 1 is placed in a CVD reactor,which is set to a low pressure of 3.9 to 13 kPa. A hydrogen carrier gasis guided into the CVD reactor, and the substrate 1 is heated in thearsine gas atmosphere up to 600 to 800 degrees centigrade. The substrate1 is kept at the temperature, and the surface thereof is cleaned. Thegroup III, group V, and doping materials are properly selected dependingon layers to be grown and are supplied at predetermined flow ratesthrough valves and/or mass flow controllers, to epitaxially grow thebuffer layer 2 up to the current block layer 91.

b) The wafer is picked out of the CVD reactor, and the current blocklayer 91 is patterned as shown in FIG. 5 according to thephotolithography technique.

c) The wafer is again set in the CVD reactor, and the LPMOCVD techniqueis employed to form a p-type Ga_(0.3)Al_(0.7)As current diffusion layer10 (N=2×10¹⁸ cm⁻³) of about 4.5 μm thick and a p-type GaAs contact layer11 (N=2×10¹⁸ cm⁻³) of about 0.1 ∥m thick. The wafer is picked out of theCVD reactor.

d) Electrode material is deposited on each side of the wafer accordingto a vacuum evaporation or a sputtering technique. The photolithographytechnique is employed to form a p-type electrode 13 on the contact layer11, and an n-type electrode 12 on the bottom surface of the substrate 1.The contact layer 11 is selectively removed according to thephotolithography technique so that the contact layer 11 is left onlyunder the electrode 13 and the current diffusion layer 10 is partlyexposed.

e) The wafer is diced into individual chips that form each the DH-LED ofFIG. 5.

(Example 1.1)

Various modifications will be possible based on the first embodiment.For example, the first n-type clad layer 44 and first p-type clad layer83 are each about 0.05 μm thick, the second n-type clad layer 43 andsecond p-type clad layer 84 are each about 0.55 μm thick, and the otherlayers are the same as those of the first embodiment. This modificationforms a φ 5-mm lamp that has an initial brightness I_(o) of 710(relative value) and a remnant brightness ratio n of 90% similar to thefirst embodiment.

(Example 1.2)

A second modification of the first embodiment employs different Al molefractions. For example, the Al mole fractions “y” and “z” of the firstn- and p-type clad layers 44 and 83 having the same thickness as that ofthe first embodiment are each 0.6. In this case, the initial brightnessI_(o) of a φ5-mm lamp to be formed is 725 (relative value) and theremnant brightness ratio η thereof is 100%. When the Al mole fractions“y” and “z” of the first clad layers 44 and 83 are each 0.8, the initialbrightness I_(o) of a φ 5-mm lamp to be formed is 720 (relative value)and the remnant brightness ratio η thereof is 95%. This modificationrealizes the maximum brightness level of the prior art and a remnantbrightness ratio greater than the prior art.

(Second embodiment)

FIGS. 6 and 7 show DH-LEDs according to the second embodiment of thepresent invention. Unlike the first embodiment that forms two cladlayers on each side of an active layer, the second embodiment forms twoclad layers only on one side of an In_(0.5)(Ga_(0.7)Al_(0.3))_(0.5)Pactive layer 61 . With this structure, the second embodiment providesthe same effect as the first embodiment.

In FIG. 6, a p-type In_(0.5)Al_(0.5)P clad layer 14 of about 0.6 μmthick is formed on the p-type side of the active layer 61, and on then-type side thereof, there are formed a first n-typeIn_(0.5)(Ga_(0.3)Al_(0.7))_(0.5)P clad layer 44 of about 0.01 μm thickand a second n-type In_(0.5)Al_(0.5)P clad layer 43 of about 0.59 μmthick.

In FIG. 7, an n-type clad layer 15 is formed on the n-type side of theactive layer 61, and on the p-type side thereof, two p-type clad layers83 and 84 are formed.

In this way, the second embodiment forms multiple clad layers only onone side of an active layer with the Al mole fraction of the multipleclad layers increasing in proportion to the distance from the activelayer, to improve the crystal characteristics of an interface betweenthem and effectively confine carriers in the active layer.

The semiconductor light emitting device of the second embodimentprovides high brightness and long service life.

(Third embodiment)

FIG. 8 is a sectional view showing a DH-LED formed on a sapphire (Al₂O₃)substrate according to the third embodiment of the present invention.Successively formed on the sapphire substrate 21 are a GaN buffer layer22, an n-type GaN layer 23, a second n-type Al_(0.2)Ga_(0.8)N clad layer24, a first n-type Al_(0.05)Ga_(0.95)N clad layer 25, an InGaN activelayer 26, a first p-type Al_(0.05)Ga_(0.95)N clad layer 27, a secondp-type Al_(0.2)Ga_(0.8)N clad layer 28, and a p-type GaN layer 29. On apart of the layer 29, a p-type electrode 13 is formed. A U-shaped trenchis formed through the layers 29, 28, 27, 26, 25, and 24 up to the layer23. On the bottom of the U-shaped trench, an n-type electrode 12 isformed. The third embodiment provides the same effect as the first andsecond embodiments.

The active layer 26 serving as a light emitting layer is sandwichedbetween the clad layers with the Al mole fraction of the clad layers 25and 27 proximal to the active layer 26 being lower than that of the cladlayers 24 and 28 distal from the active layer 26. This structureimproves the crystal quality of each interface between the active layerand the clad layers, to effectively confine carriers in the active layer26. As a result, the light emitting device of the third embodimentprovides high brightness and long service life.

Although the third embodiment forms two clad layers on each side of theactive layer 26, the two clad layers may be formed only on one side ofthe active layer, to provide the same effect.

(Fourth embodiment)

FIG. 9 is a sectional view showing a DH-LED according to the fourthembodiment of the present invention. Similar to the first embodiment,the fourth embodiment forms each two p- and n-type clad layers. Namely,a first n-type In_(0.5)(Ga_(1−y)Al)_(0.5)P clad layer 46 and a firstp-type In_(0.5)(Ga_(1−y)Al_(y))_(0.5)P clad layer 85 are formed onopposite sides of an In_(0.5)(Ga_(1−y)Al_(y))_(0.5)P active layer 61where

x+0.1≦y≦x+0.5

and a second n-type In_(0.5)(Ga_(1−z)Al_(z))_(0.5)P clad layer 45 and asecond p-type In_(0.5)(Ga_(1−z)Al_(z))_(0.5)P clad layer 86 are formedoutside the layers 46 and 85, respectively, where the Al mole fraction“z” is larger than “y.” The thickness of each of the first clad layers46 and 85 is in the range of 0.005 to 0.1 μm.

The Al mole fraction “x” of the active layer 61 may take 0, and the Almole fraction “z” of the second clad layers may take 1. In the followingexplanation, the active layer is made ofIn_(0.5)(Ga_(0.72)Al_(0.28))_(0.5)P, and the second clad layers 45 and86 are made of n-type In_(0.5)Al_(0.5)P and p-type In_(0.5)Al_(0.5)P,respectively. The present invention, however, is not limited to thesecompounds.

The DH-LED of FIG. 9 employs an n-type GaAs substrate 1 having animpurity concentration of 0.4×10¹⁸ to 3×10¹⁸ cm⁻¹. Successively formedon the substrate 1 are an n-type GaAs buffer layer 2, a reflection layer3 composed of 10 pairs of alternating n-type In_(0.5)Al_(0.5)P film andn-type GaAs film, the second n-type In_(0.5)Al_(0.5)P clad layer 45, thefirst n-type In_(0.5)(Ga_(0.4)Al_(0.6))_(0.5)P clad layer 46, theundoped n-type In_(0.5)(Ga_(0.72)Al_(0.28))_(0.5)P active layer 61, thefirst p-type In_(0.5)(Ga_(0.4)Al_(0.6))_(0.5)P clad layer 85, the secondp-type In_(0.5)Al_(0.5)P clad layer 86, a p-type In_(0.5)Ga_(0.6)Pcontact layer 87, a p-type GaAs protection 15 layer 50, an n-typeIn_(0.5)(Ga_(0.3)Al_(0.7))_(0.5)P current block layer 51, an n-type GaAsprotection layer 52, a p-type Ga_(0.3)Al_(0.7)As current diffusion layer10, a p-type In_(0.5)(Ga_(0.7)Al_(0.3))_(0.5)P moisture resistant layer55, and a p-type GaAs contact layer 11. On the contact layer 11, ap-type electrode 13 is formed from Au—Zn. Over the bottom surface of thesubstrate 1, an n-type electrode 12 is formed from Au-Ge. The electrode13 and contact layer 11 are each of about 140 μm in diameter and areformed at the center of the moisture resistant layer 55. The surface ofthe moisture resistant layer 55 except the part where the electrode 13is formed emanates light from the active layer 61.

The active layer 61 serving as a light emitting layer is indirectly incontact with the second n- and p-type clad layers 45 and 86 whose Almole fractions are greatly differ from that of the active layer 61, withthe first n- and p-type clad layers 46 and 85 being interposed betweenthem. Here, the Al mole fraction “y” of the first clad layers 46 and 85and the Al mole fraction “x” of the active layer 61 are designed as“x+0.1≦y≦x+0.5” and the thickness of each of the first clad layers 46and 85 is in the range of 0.005 to 0.1 μm. This structure improves thecrystal quality of each interface between the active layer 61 and theclad layers. Accordingly, the DH-LED of the fourth embodiment maintainsan initial brightness level equivalent to that of the prior art whoseservice life is short as shown in FIG. 10A, and secures longer servicelife as shown in FIG. 10B than the prior art of FIG. 4.

The processes of manufacturing the DH-LED of the fourth embodiment willbe explained with reference to FIGS. 11A to 11E.

(a) In FIG. 11A, the LPMOCVD technique is used to successively form onan n-type GaAs substrate 1 an Si-doped n-type GaAs buffer layer 2 ofabout 0.5 μm thick with an impurity concentration N of 2.0 to 5.0×10¹⁷cm⁻³, a reflection layer 3 composed of 10 pairs of alternating Si-dopedn-type In_(0.5)Al_(0.5)P film (N=0.4 to 2.0×10¹⁸ cm⁻³) of about 40 nmthick and Si-doped n-type GaAs film (N=0.4 to 2.0×10¹⁸ cm⁻³) of about 40nm thick, an Si-doped second n-type In_(0.5)Al_(0.5)P clad layer 45(N=2.0 to 6.0×10¹⁷) of about 0.6 μm thick, an Si-doped first n-typeIn_(0.5)(Ga_(0.4)Al_(0.6))_(0.5)P clad layer 46 of about 0.05 μm thick,an undoped n-type In_(0.5)(Ga_(0.72)Al_(0.28))_(0.5)P active layer 61 (Nis approximately 1.0×10¹⁷ cm⁻³) of about 0.6 μm thick, a Zn-doped firstp-type In_(0.5)(Ga_(0.4)Al_(0.6))_(0.5)p clad layer 85 of about 0.05 μmthick, a Zn-doped second p-type In_(0.5)Al_(0.5)P clad layer 86 (N=2.0to 6.1×10¹⁷ cm⁻³) of about 0.6 μm thick, a Zn-doped p-typeIn_(0.5)Ga_(0.5)P contact layer 87 (N=0.5 to 1.0×10¹⁸ cm⁻³) of about0.02 μm thick, a Zn-doped p-type GaAs protection layer 50 (N=1.0 to6.0×10¹⁸ cm⁻³) of about 0.01 μm thick, an Si-doped n-typeIn_(0.5)(Ga_(0.3)Al_(0.7))_(0.5)P current block layer 51 (N=0.5 to1.0×10¹⁹ cm⁻³) of about 0.02 μm thick, an Si-doped n-type GaAsprotection layer 52 (N=1.0 to 6.0×10¹⁸ cm⁻³) of about 0.01 μm thick, andan Si-doped n-type In_(0.5)(Ga_(0.3)Al_(0.7))_(0.5)P cap layer 53 (N=1.0to 6.0×10¹⁸ cm⁻³) of about 0.02 μm thick.

(b) In FIG. 11B, the cap layer 53 and protection layer 52 areselectively etched into a required shape to partly expose the currentblock layer 51 according to a wet etching technique.

(c) In FIG. 11C, the cap layer 53 and the current block layer 51 areetched according to the wet etching technique, to partly expose theprotection layer 50.

(d) In FIG. 11D, the LPMOCVD technique is used to sequentially form onthe wafer of FIG. 11C a Zn-doped p-type Ga_(0.3)Al_(0.7)As currentdiffusion layer 10 (N=1.0 to 6.0×10¹⁸ cm⁻³) of about 4.5 μm thick, aZn-doped p-type In_(0.5)(Ga_(0.7)Al_(0.3))_(0.5)P moisture resistantlayer 55 (N=0.5 to 2.0×10¹⁸ cm⁻³) of about 0.1 μm thick, a Zn-dopedp-type GaAs contact layer 11 (N=1.0 to 6.0×10¹⁸ cm⁻³) of about 0.1 μmthick, and an Si-doped n-type In_(0.5)(Ga_(0.3)Al_(0.7))_(0.5)P caplayer 57 (N=1.0 to 6.0×10¹⁸ cm⁻³) of about 0.15 μm thick.

(e) In FIG. 11E, the bottom surface of the substrate 1 is removed by 5μm according to the wet etching technique. The cap layer 57 is entirelyremoved according to the wet etching technique. An n-type electrode 12is formed from Au-Ge on the bottom surface of the substrate 1. A p-typeelectrode 13 of about 140 μm in diameter is formed from Au-Zn on thecontact layer 11 Just above the current block layer 51. Exposed part ofthe contact layer 11 is removed according to the wet etching technique.The wafer is diced by blade dicing into chips each having a side lengthof about 320 μm. Damaged (or broken) layers on each side face of thechip caused by the blade dicing are removed according to the wet etchingtechnique, to complete each light emitting device.

The structures, materials, and dimensions mentioned above are onlyexamples. The first clad layers 46 and 85 of FIG. 9 may have thefollowing thicknesses and Al mole fractions to provide the same effect:

(a) The first n-type clad layer 46 is an Si-doped n-typeIn_(0.5)(Ga_(0.2)Al_(0.2))_(0.5)P layer of about 0.05 μm thick, and thefirst p-type clad layer 85 is a Zn-doped p-typeIn_(0.5)(Ga_(0.2)Al_(0.6))_(0.5)P layer of about 0.05 μm thick.

(b) The first n-type clad layer 46 Is an Si-doped n-typeIn_(0.5)(Ga_(0.4)Al_(0.6))_(0.5)P layer of about 0.01 μm thick, and thefirst p-type clad layer 85 is a Zn-doped p-typeIn_(0.5)(Ga_(0.4)Al_(0.6))_(0.5)P layer of about 0.01 μm thick.

(c) The first n-type clad layer 46 is an Si-doped n-typeIn_(0.5)(Ga_(0.2)Al_(0.8))_(0.5)P layer of about 0.10 μm thick, and thefirst p-type clad layer 85 is a Zn-doped p-typeIn_(0.5)(Ga_(0.2)Al_(0.8))_(0.5)P layer of about 0.10 μm thick.

Even if one of the first clad layers 85 and 46 is omitted, the effect ofthe present invention will be secured. For example, the following arepossible:

(d) Only the first n-type clad layer 46 of about 0.05 μm thick is formedfrom Si-doped n-type In_(0.5)(Ga_(0.4)Al_(0.6))_(0.5)P.

(e) Only the first p-type clad layer 85 of about 0.05 μm thick is formedfrom Zn-doped p-type In_(0.5)(Ga_(0.4)Al_(0.6))_(0.5)P.

(Fifth embodiment)

FIGS. 12A and 12B show a DH-LED according to the fifth embodiment of thepresent invention. This embodiment gradually increases the Al molefraction of a first clad layer. Sequentially formed on an n-type GaAssubstrate 1 are an n-type GaAs buffer layer 2, an n-type InAlP/GaAsreflection layer 3, a second n-type InGaAlP clad layer 43, a firstn-type InGaAlP clad layer 48, an undoped InGaAlP active layer 61, afirst p-type InGaAlP clad layer 88, and a second p-type InGaAlP cladlayer 84. On a part of the second clad layer 84, an n-type GaAs currentblock layer 91 is formed. On the current block layer 91, a p-type GaAlAscurrent diffusion layer 10 and a p⁺-type GaAs contact layer 11 areformed. The reflection layer 3 has a multilayer structure composed of 10to 20 pairs of alternating n-type InAlP layer and n-type GaAs layerhaving a thickness of λ×¼ n where λ is the wavelength of light emittedfrom the active layer 61 and n is the refractive index of the materialof the layers.

The Al mole fractions of the clad layers 48, 43, 88, and 84 and currentdiffusion layer 10 are determined to sufficiently transmit the lightfrom the active layer 61. On the bottom surface of the substrate 1, ann-type electrode 12 is formed entirely. On the contact layer 11, ap-type electrode 13 is formed. The diameter of the electrode 13 is about140 p m so that the light from the active layer 61 emanates from aroundthe electrode 13. FIG. 12B shows a profile of Al mole fractions of theclad layers. The Al mole fraction of the first clad layers 48 and 88gradually linearly increases in proportion to the distance from theactive layer 61.

The Al mole fraction “x” of the second n-typeIn_(0.5)(Ga_(1−x)Al_(x))_(0.5)P clad layer 43 is fixed at 0.8. The Almole fraction “y” of the first n-type In_(0.5)(Ga_(1−y)Al_(y))_(0.5)Pclad layer 48 gradually decreases from 0.8 to 0.3 from the second n-typeclad layer 43 toward the active layer 61. The Al mole fraction “z” ofthe first p-type In_(0.5)(Ga_(1−y)Al_(y))_(0.5)P clad layer 88 graduallyincreases from 0.3 to 0.8 from the active layer 61 toward the secondp-type clad layer 84. The Al mole fraction “w” of the second p-typeIn_(0.5)(Ga_(1−x)Al_(x))_(0.5)P clad layer 84 is fixed at 0.8. Thethickness of each of the second clad layers 43 and 84 is 0.59 μm andthat of each of the first clad layers 48 and 88 is 0.1 μm.

The DH-LED of FIG. 12a is mounted on a stem, bonded, and sealed withresin, to form a φ 5-mm lamp. The initial brightness I_(o) of this lampis equal to the maximum brightness of the prior art, and the remnantbrightness ratio (η=I₁₀₀₀/I_(o)) of brightness I₁₀₀₀ after 1000 hours ofoperation at 50 mA to the initial brightness I_(o) is 85% that is fargreater than the prior art.

The high quality of the fifth embodiment is derived from the two cladlayers on each side of the active layer 61, with the Al mole fraction ofthe first clad layers 48 and 88 being low in the vicinities of theactive layer 61 and gradually increasing in proportion to the distancefrom the active layer 61, to improve the crystal quality of eachinterface between the clad layers 48 and 88 and the active layer 61. TheAl mole fractions of the first clad layers 48 and 88 are y=0.8 and z=0.8at positions where the layers 48 and 88 are in contact with the secondclad layers 43 and 84 whose Al mole fractions are each fixed at 0.8 tosufficiently confine carriers in the active layer 61. Consequently, theDH-LED of the fifth embodiment provides high brightness and long servicelife.

The DH-LED of the fifth embodiment is manufactured according to the sameprocesses as those of the first embodiment. On an n⁺-type GaAs substrate1, an n-type GaAs buffer layer 2 of about 0.5 μm thick having animpurity concentration N=4×10¹⁷ cm⁻³ is formed. On the buffer layer 2, areflection layer 3 is formed by alternately growing 10 to 20 pairs ofn-type In_(0.5)Al_(0.5)P film (N=4×10¹⁷ cm⁻³) and GaAs film (N=4×10¹⁷cm⁻³) of λ/4n thick according to the LPMOCVD technique. Successivelygrown on the reflection layer 3 are a second n-typeIn_(0.5)(Ga_(1−x)Al_(x))_(0.5)P clad layer 43 (N=4×10¹⁷ cm⁻³, x=1.0) ofabout 0.59 μm thick and a first n-type In_(0.5)(Ga_(1−y)Al_(y))_(0.5)Pclad layer 48 (N=4×10¹⁷ cm⁻³) of about 0.01 μm thick with the Al molefraction “y” being gradually decreased. Decreasing the Al mole fraction“y” from 0.8 to 0.3 is achieved by relatively decreasing the flow rateof TMA with respect to the flow rates of TMI, TMG, and PH3 with the useof a program-controlled mass flow controller. On the clad layer 48, anundoped In_(0.5)(Ga_(0.7)Al_(0.3))_(0.5)P active layer 61 of about 0.6μm thick is formed. On the active layer 61, a first p-typeIn_(0.5)(Ga_(1−z)Al_(z))_(0.5)P clad layer 88 (N=4×10¹⁷ cm⁻³) of about0.01 μm thick is formed with the Al mole fraction “z” being graduallyincreased from 0.3 to 0.8 with the use of the mass flow controller thatcontrols the flow rate of TMA according to a program. On the clad layer88, a second p-type In_(0.5)(Ga_(0.2)Al_(0.6))_(0.5)P clad layer 84(N=4×10¹⁷ cm⁻³) of about 0.59 μm thick and an n-type GaAs current blocklayer 91 (N=2×10¹⁸ cm⁻³) of about 0.02 μm thick are successively formedaccording to the LPMOCVD technique. The other processes are the same asthose of the first embodiment, and therefore, are not explained again.

Modifications of the fifth embodiment will be explained.

(Example 5.1)

FIG. 13A shows change in the Al mole fractions of clad layers of aDH-LED according to a first modification of the fifth embodiment.Successively formed on an n⁺-type GaAs substrate 1 are an n-type GaAsbuffer layer 2, an n-type InAlP/GaAs reflection layer 3, a second n-typeIn_(0.5)(Ga_(0.2)Al_(0.5))_(0.5)P clad layer 43, a first n-typeIn_(0.5)(Ga_(1−y)Al_(y))_(0.5)P clad layer 48, an undopedIn_(0.5)(Ga_(0.7)Al_(0.3))_(0.5)P active layer 61, a first p-typeIn_(0.5)(Ga_(1−z)Al_(z))_(0.5)P clad layer 88, and a second p-typeIn_(0.5)(Ga_(0.2)Al_(0.8))_(0.5)P clad layer 84. On a part of the cladlayer 84, an n-type GaAs current block layer 91 is formed, and on thecurrent block layer 91, a p-type GaAlAs current diffusion layer 10 and ap⁺-type GaAs contact layer 11 are sequentially formed.

Each of the first clad layers 48 and 88 has a multilayer structure.Namely, the first n-type clad layer 48 is composed of four films, i.e.,n-type In_(0.5)(Ga_(0.3)Al_(0.7))_(0.5)P film, n-typeIn_(0.5)(Ga_(0.4)Al_(0.6))_(0.5)P film, n-typeIn_(0.5)(Ga_(0.5)Al_(0.5))_(0.5)P film, and n-typeIn_(0.5)(Ga_(0.6)Al_(0.4))_(0.5)P film each of 30 nm thick. In this way,the Al mole fraction “y” of these films decreases from the clad layer 43toward the active layer 61. The first p-type clad layer 88 is composedof four films, i.e., p-type In_(0.5)(Ga_(0.6)Al_(0.4))_(0.5)P film,p-type In_(0.5)(Ga_(0.5)Al_(0.5))_(0.5)P film, p-typeIn_(0.5)(Ga_(0.4)Al_(0.6))_(0.5)P film, and p-typeIn_(0.5)(Ga_(0.3)Al_(0.7))_(0.5)P film each of 30 nm thick. In this way,the Al mole fraction “z” of these films increases from the active layer61 toward the clad layer 84.

The first clad layers 48 and 88 sandwiching the active layer 61 betweenthem have each a multilayer structure with the Al mole fraction thereofincreasing from the active layer 61 toward the second clad layers 43 and84, to improve the crystal quality of each interface between the activelayer 61 and the first clad layers 48 and 88. The Al mole fraction ofthe second clad layers 43 and 84 is high to effectively confine carriersin the active layer 61. The DH-LED of this modification, therefore,provides high brightness and long service life.

(Example 5.2)

FIG. 13B shows change in the Al mole fraction of clad layers of a DH-LEDaccording to a second modification of the fifth embodiment. Successivelyformed on an n⁺-type GaAs substrate 1 are an n-type GaAs buffer layer 2,an n-type InAlP/GaAs reflection layer 3, a second n-typeIn_(0.5)Al_(0.5)P clad layer 43, a first n-typeIn_(0.5)(Ga_(1−y)Al_(y))_(0.5)P clad layer 48, an undopedIn_(0.5)(Ga_(0.6)Al_(0.2))_(0.5)P active layer 61, a first p-typeIn_(0.5)(Ga_(1−z)Al_(z))_(0.5)P clad layer 88, and a second p-typeIn_(0.5)Al_(0.5)P clad layer 84.

On a part of the second clad layer 84, an n-type GaAs current blocklayer 91 is formed, and on the current block layer 91, a p-type GaAlAscurrent diffusion layer 10 and a p⁺-type GaAs contact layer 11 aresequentially formed.

The Al mole fraction “y” of the first n-type clad layer 48 linearlydecreases from 0.8 to 0.3 from the clad layer 43 toward the active layer61. The Al mole fraction “z” of the first p-type clad layer 88 linearlyincreases from 0.3 to 0.8 from the active layer 61 toward the clad layer84. Unlike the profile of FIG. 12B, the profile of the Al mole fractionof FIG. 13B has a step at each interface between the active layer 61 andthe first clad layers 48 and 88 and at each interface between the firstclad layers 48 and 88 and the second clad layers 43 and 84. In spite ofthis, the structure of FIG. 13B provides substantially the same effectas the fifth embodiment of FIG. 12A.

In this way, the second modification of FIG. 13B arranges the cladlayers 48 and 43 on one side of the active layer 61 and the clad layers88 and 84 on the other side thereof. The Al mole fraction of the firstclad layers 48 and 88 linearly changes from 0.3 to 0.8 outwardly fromthe active layer 61, to decrease the Al mole fraction in each interfacealong the active layer 61 and improve the crystal quality of theinterfaces. The Al mole fraction of the second clad layers 43 and 84distal from the active layer 61 is high to sufficiently confine carriersin the active layer 61. As a result, the DH-LED of the secondmodification provides high brightness and long service life.

(Example 5.3)

FIG. 13C shows change in the Al mole fraction of clad layers of a DH-LEDaccording to the third modification of the fifth embodiment. Thismodification is characterized in that it changes the Al mole fraction ofonly one first clad layer between 0.3 and 0.8. This structure is capableof providing the same effect as that of FIG. 12A.

The Al mole fraction of a first n-type clad layer 44 is constant(y=0.7), and that (z) of a first p-type clad layer 88 increases from 0.3to 1.0 in proportion to the distance from an active layer 61.Alternatively, the Al mole fraction “z” of the first p-type clad layer88 may be constant, and that (y) of the first n-type clad layer 44 maygradually be increased in proportion to the distance from the activelayer 61.

In this way, the third modification linearly changes the Al molefraction of one of the first clad layers that sandwich the active layerso that the Al mole fraction of the layer may increase in proportion tothe distance from the active layer, to improve the crystalcharacteristics of an interface between the active layer and the cladlayer and efficiently confine carriers in the active layer. The DH-LEDof the third modification, therefore, provides high brightness and longservice life.

(Example 5.4)

FIG. 13D shows change in the Al mole fraction of clad layers of a DH-LEDaccording to a fourth modification of the fifth embodiment. Thismodification is characterized in that multiple clad layers are formed ononly one side of an active layer similar to the second embodiment ofFIGS. 6 and 7 and in that the Al mole fraction of a first one of themultiple clad layers is linearly changed. This structure provides thesame effect as the fifth embodiment.

The DH-LED of FIG. 13D has a p-type In_(0.4)Al_(0.6)P clad layer 14 ofabout 0.6 μm thick, a first n-type In_(0.5)(Ga_(1−y)Al_(y))_(0.5)P cladlayer 48 of about 0.01 μm thick whose Al mole fraction “y” varies, and asecond n-type In_(0.5)(Ga_(0.2)Al_(0.8))_(0.5)P clad layer 43 of about0.59 μm thick. Instead, the DH-LED may have one n-type clad layer andtwo p-type clad layers 88 and 84 as shown in FIG. 12A.

In this way, the fourth modification forms two clad layers only on oneside of an active layer with the Al mole fraction of a first one of thetwo clad layers being linearly changed from a low level in the vicinityof the active layer to a high level at a distal end of the clad layer.This arrangement improves the crystal quality of an interface betweenthe active layer and the first clad layer and effectively confinecarriers in the active layer. The DH-LED of the fourth modificationprovides high brightness and long service life.

Various modifications will become possible for those skilled in the artafter receiving the teachings of the present disclosure withoutdeparting from the scope thereof. Although the first to fifthembodiments employ first and second clad layers, it is possible toarrange more clad layers outside the second clad layers. The presentinvention is applicable to InGaAlP-based LEDs for emitting visible lightof from green to red, GaN-based LEDs for emitting visible light of blue,and GaAs-based LEDs for emitting infrared light.

The clad layers may be made of mixed crystals of groups III and V suchas InGaN and InGaAsP, or mixed crystals of groups II and VI such asZnSeTe, MgSeTe, ZnSSe, and MgSSe.

The fifth embodiment linearly changes the Al mole fractions “y” and “z”of the first clad layers 48 and 88. These Al mole fractions may bechanged according to a quadratic curve, a cubic curve, or an exponentialcurve. Gradually increasing and decreasing Al mole fractions of thefifth embodiment may be combined with any one of the third and fourthembodiments. When the fifth embodiment is combined with the fourthembodiment, the Al mole fraction “y” of the first n- and p-type cladlayers is gradually increased and decreased with respect to the Al molefraction “x” of the active layer in the range of x+0.1≦y≦x+0.5.

The DH-LEDs of the present invention provide high brightness and longservice life, and therefore, are applicable to indoor and outdoordisplay panels. The LEDs may be arranged in an X-Y matrix fashion toform an LED matrix circuit, or the LED matrix circuit may be arrangedside by side to form an LED module panel. FIG. 14A shows an example ofthe LED matrix circuit having φ 5-mm to φ10-mm lamps 516 based on anyone of the embodiments of FIGS. 5 to 9 and 12A. The red and green lamps(or LEDs) are arranged in a 32-by-32 matrix serving as dots. The LEDmatrix circuit of FIG. 14A has a matrix of 32 data lines D1 to D32 and32 scan lines C1 to C32 with the LEDs 516 being connected to theintersections of the data and scan lines. Two sets (not shown) of thematrix of 32 data lines and 32 scan lines of FIG. 14A are arranged forthe red and green LED groups, respectively.

Each LED lamp 516 may have a current control resistor 517 as shown inFIG. 14B. The resistor 517 adjusts a current flowing to the LED lamp516, to equalize the brightness of light among the LED lamps 516. Twosets (not shown) of the matrix of 32 data lines D1, D2, . . . , D32 and32 scan lines C1, C2, . . . , C32 of FIG. 14B are arranged for the redand green LED groups, respectively. The LED matrix circuit (ordot-matrix circuit) of any one of FIGS. 14A and 14B is used to fabricatean outdoor/indoor LED display apparatus of FIG. 15.

An LED module panel 705 of the apparatus of FIG. 15 has a plurality ofLED matrix circuits each being any one of FIGS. 14A and 14B, to providea large screen. An external computer 712 incorporates a video captureunit 701 that provides an analog RGB signal. An A/D converter 702converts the signal into a digital RGB signal, which is processed by aninterface 703 into a display data signal. The display data signal issupplied to the LED matrix circuits of the panel 705 through a buffer704. The LED lamps of the LED matrix circuits emit light in response tothe display data signal, to display an image having gradations. Each LEDof the LED matrix circuits of the apparatus of FIG. 15 has good crystalquality and effectively confines carriers due to the DH structure.Accordingly, the apparatus of FIG. 15 is efficient, has long servicelife, is visible from a long distance, and is reliable.

What is claimed is:
 1. A light emitting diode for emitting spontaneousradiation and having a double heterostructure, comprising: (a) an activelayer for emitting the spontaneous radiation having a given wavelength;b) a first clad layer of a first conductivity type formed on the activelayer; c) a second clad layer, separate from the other structures, ofthe first conductivity type formed on the first clad layer of the firstconductivity type; d) a semiconductor layer uniformly formed on a wholesurface of the second clad layer of the first conductivity type; e) anupper metal electrode formed over a part of the semiconductor layer soas to define an optical aperture for emitting upward the spontaneousradiation from said active layer through said first and second cladlayer of the first conductivity type and the semiconductor layer; f) afirst clad layer of a second conductivity type formed under the activelayer; g) a second clad layer separate from other structures of thesecond conductivity type formed under the first clad layer of the secondconductivity type; and h) a substrate formed under the second clad layerof the second conductivity type, a forbidden band gap of the first cladlayer of the first conductivity type being smaller than that of thesecond clad layer of the first conductivity type, and the forbidden bandgap of the first clad layer of the second conductivity type beingsmaller than that of the second clad layer of the second conductivitytype.
 2. The diode of claim 1, further comprising: a U-shaped trenchpenetrating said semiconductor layer, said first and second clad layerof the first conductivity type, said active layer, and said first cladlayer of the second conductivity type; and a lower metal electrodeformed on a part of said second clad layer of the second conductivitytype.
 3. The diode of claim 2, wherein the substrate is made ofsapphire.
 4. The diode of claim 1, wherein the active layer is made ofGaN based semiconductor.
 5. The diode of claim 4, wherein the activelayer is made of InGaN.
 6. The diode of claim 5, wherein the first andthe second clad layer of the first conductivity type are made of AlGaN.7. The diode of claim 6, wherein the first clad layer of the firstconductivity type is made of p-type Al_(0.05)Ga_(0.95)N and the secondclad layer of the first conductivity type is made of p-typeAl_(0.2)Ga_(0.8)N.
 8. The diode of claim 5, wherein the first and thesecond clad layer of the second conductivity type are made of AlGaN. 9.The diode of claim 8, wherein the first clad layer of the secondconductivity type is made of n-type Al_(0.05)Ga_(0.95)N and the secondclad layer of the second conductivity type is made of n-typeAl_(0.2)Ga_(0.8)N.
 10. The diode of claim 1, wherein the substrate ismade of sapphire.
 11. The diode of claim 1, wherein the Al molefractions of the first clad layers of the first and second conductivitytype gradually increase in proportion to the distance from the activelayer, respectively.
 12. The diode of claim 1, wherein the thickness ofeach of the first clad layers of the first and second conductivity typesis in the range of 0.005 to 0.1 μm, and the thickness of each of thesecond clad layers of the first and second conductivity types is in therange of 0.005 to 0.1 μm.
 13. The diode of claim 12, wherein the totalthickness of the first and second clad layers of the first conductivitytype is 0.05 to 1.0 μm, and the total thickness of the first and secondclad layers of the second conductivity type is 0.05 to 0.1 μm.
 14. Alight emitting diode for emitting a spontaneous radiation having adouble heterostructure comprising: a) an active layer for emitting thespontaneous radiation having a given wavelength; b) a first clad layerof a first conductivity type formed on the active layer; c) a secondclad layer, separate from other structures of the first conductivitytype formed on the first clad layer of the first conductivity type; d) asemiconductor layer formed on the second clad layer of the firstconductivity type; e) an upper metal electrode formed over a part of thesemiconductor layer so as to define an optical aperture for emittingupward the spontaneous radiation from said active layer through saidfirst and second clad layer of the first conductivity type and thesemiconductor layer; f) a first clad layer of a second conductivity typeformed under the active layer; g) a second clad layer, separate fromother structures of the second conductivity type formed under the firstclad layer of the second conductivity type; h) a substrate formed underthe second clad layer of the second conductivity type; i) a U-shapedtrench penetrating said semiconductor layer, said first and second cladlayer of the first conductivity type, said active layer, and said firstclad layer of the second conductivity type down to said second cladlayer of the second conductivity type; and j) a lower metal electrodeformed on a part of said second clad layer of the second conductivitytype, a forbidden band gap of the first clad layer of the firstconductivity type being smaller than that of the second clad layer ofthe first conductivity type, and the forbidden band gap of the firstclad layer of the second conductivity type being smaller than that ofthe second clad layer of the second conductivity type.
 15. The diode ofclaim 14, wherein the active layer is made of GaN based semiconductor.16. The diode of claim 15, wherein the active layer is made of InGaN.17. The diode of claim 16, wherein the first and the second clad layerof the first conductivity type are made of AlGaN.
 18. The diode of claim17, wherein the first clad layer of the first conductivity type is madeof p-type Al_(0.05)Ga_(0.95)N and the second clad layer of the firstconductivity type is made of p-type Al_(0.2)Ga_(0.8)N.
 19. The diode ofclaim 16, wherein the first and the second clad layer of the secondconductivity type are made of AlGaN.
 20. The diode of claim 19, whereinthe first clad layer of the second conductivity type is made of n-typeAl_(0.05)Ga_(0.95)N and the second clad layer of the second conductivitytype is made of n-type Al_(0.2)Ga_(0.8)N.
 21. The diode of claim 14,wherein the substrate is made of sapphire.
 22. A light emitting diodefor emitting a spontaneous radiation and having a doubleheterostructure, comprising: a) an active layer for emitting thespontaneous radiation having a given wavelength; b) a first clad layerof a first conductivity type formed on the active layer; c) a secondclad layer, separate from other structures, of the first conductivitytype formed on the fist clad layer of the first conductivity type; d) asemiconductor layer uniformly formed on a whole surface of the secondclad layer of the first conductivity type; e) an upper metal electrodeformed over a part of the semiconductor layer so as to define an opticalaperture for emitting upward the spontaneous radiation from said activelayer through said first and second clad layer of the first conductivitytype and the semiconductor layer; f) a first clad layer of a secondconductivity type formed under the active layer; g) a second clad layer,separate from other structures, of the second conductivity type formedunder the first clad layer of the second conductivity type; and h) asubstrate formed under the second clad layer of the second conductivitytype, wherein a forbidden band gap of the first clad layer of the firstconductivity type being smaller than that of the second clad layer ofthe first conductivity type, and the forbidden band gap of the firstclad layer of the second conductivity type being smaller than that ofthe second clad layer of the second conductivity type, and wherein theactive layer, the first and second clad layers of the first conductivitytype, and the first and second clad layers of the second conductivitytype are each made of compound semiconductor containing Al, the Al molefraction of the active layer is smaller than that of the first cladlayers of the first and second conductivity types, and the Al molefraction of the second clad layers of the first and second conductivityis larger than that of the first clad layers of the first and secondconductivity type.
 23. The diode of claim 22, wherein the Al molefraction of the active layer includes zero.